Structural concrete slabs often require protection from heaving caused by the upward movement of underlying ground. The preferred approach is to create a void space under the concrete slab. These concrete slabs can be supported at intervals on foundations and void forming materials are used to create void spaces between the concrete slab and the ground. Void forming materials must be able to support the weight of fresh concrete, as well as live loads provided by workmen and equipment during the course of construction. At the completion of construction, the void forming materials must begin to break down in order to protect the concrete from heaving ground.
Void spaces have been constructed using wax-coated cardboard honeycomb products that degrade when exposed to moisture. However, jobsite delays or inclement weather can result in premature strength loss of the cardboard product, causing collapse of the concrete before it is self supporting.
Foam plastic materials, such as expanded polystyrene (EPS) have been substituted for cardboard products, as EPS is not susceptible to the elements. However, used as a compressible inclusion, the stress required to deform the EPS is higher than most concrete slabs can withstand. Various attempts have been made to overcome the problem through the use of light density EPS, resulting in lower compressive strength, and also by creating various shaped voids and configurations in the EPS to reduce the cross sectional area in contact with the soil or the slab. All of these efforts have failed to reduce the pressure on the concrete slab to a level that the slab can withstand.
The industry has, therefore, been searching for many years for a serviceable void forming material for the construction of structural concrete slabs.